The purpose of the proposed research is to characterize the cellular electrophysiology of subsidiary packmaker activity within a particular region of the dog right atrium. An in vivo right atrial preparation, perfused through its nutritive SA node artery, will be used as a model for eliciting subsidiary atrial pacemaker activity. Ligation of the SA node artery, just caudal to the SA node region, leads to suppression of pacemaker activity in the SA node region and the emergence of subsidiary pacemaker activity at the junction of the inferior right atrium - infterior vena cava. The site of subsidiary atrial pacemaker activity will be located by using bipolar extracellular recordings to estimate the epicardial site of earliest activation. Once the region of tissue exhibiting subsidiary pacemaker activity is located, it will be excised, transferred to a smaller tissue bath and superfused with Tyrode's solution. Transmembrane action potentials and a single extracellular electrogram will be used to determine the pacemaker cells having the earliest activation and to measure impulse conduction to the surrounding atrial tissue. Subsidiary pacemaker characteristics that will be determined include: 1) action potential configurations, 2) the response to autonomic neuromediators (ACh and NE) and antagonists, 3) refractory characteristics, 4) overdrive suppression, 5) pacemaker phase relationship, 6) the infuence of electrotonic interactions, 7) triggered activity, 8) the response to changes in potassium, 9) the response to ionic channel blocking agents (TTX, verapamil, nefedipine). The results from these experiments should contribute to our fundamental understanding of subsidiary pacemaker function in the right atrium and thereby to a better understanding of atrial dysrhythmias possibly resulting from their activity.